Radial piston pumps or motors, as mostly constructed, have a centrally arranged shaft driving a cam, which drives a number of radially arranged pistons. Also a reversed arrangement is known, see U.S. Pat. No. 3,087,437 to Henrichsen, where a centrally arranged pintle valve is for supplying and exhausting fluid to and from a number of cylinders in a rotating cylinder block, each cylinder including a reciprocating piston. The pistons are driven to and fro by an excentrically located cam ring, which surrounds the rotating cylinder block.
In a further known radial piston machine (pump or motor) of this kind (British patent 1 468 658, inventor: Kenneth Persival Palmer, assignee: Lucas Ltd.), the piston head is spherically shaped and has a ring groove in a certain distance to its equator plane, a piston ring being inserted in the ring groove. When the cylinder block is rotating, the pistons tilt and become inclined relatively to the axis of the cylinder bores so that also the piston rings will be oblique. One side of each piston ring travels further out of the ring groove, whereas the other side is further pressed into the ring groove. The piston ring has radially inner and outer edges, which engage the cylinder wall and produce end pressures, the end pressure at the side of the piston ring, which is shifted into the ring groove, is especially great and practically corresponds to the force, which produces the torque of the machine for the respective piston, having in mind the distance to the machine axis. Therefore, a high end pressure is met at this edge. On the other side of the piston ring, where it projects from the ring groove, this cantilevering portion under hydraulic pressure is under unfavourable bending stresses. Radial piston maschines of the type of the above named GB-A 1 468 658, therefore, cannot be met on the market.